The invention relates to a method for selectively imaging a substrate, and more particularly to a method for using an ink jet printer to selectively image a lithographic printing plate provided with a negative working photosensitive coating.
To make a typical lithographic printing plate, a substrate is provided with a hydrophilic surface which is imaged with an oleophilic material corresponding to the pattern to be printed. The oleophilic material is typically a photosensitive photopolymer or diazo resin which is exposed to light by masking with a film negative. The exposed portions are rendered insoluble in a developer and the unexposed portions remain soluble. Insolubility is typically caused by cross-linking of the photopolymer, the rate of cross-linking enhanced by coating the photopolymer with an oxygen inhibition layer. After exposure the oxygen inhibition layer and the unexposed photopolymer are removed. Other negative working materials useful in this invention include diazo resins mixed with non-photosensitive polymers and resins.
The above described process is referred to as a negative working process because the unexposed material is removed. In a positive working process, the pattern to be printed is masked and the photosensitive exposed material is rendered soluble in a developer. Printing plates made according to either process are used on offset presses. These printing presses sequentially apply water and then ink to the plates. The inked plates then transfer a right reading image to a blanket cylinder producing a wrong reading image which is then transferred to paper right reading again.
Many variations in plate manufacturing processes using photopolymers, diazo resins, mixtures and various combinations are known. The plates generally have images that are planographic, i.e. substantially flat, but they may have raised images for relief printing or intaglio images for gravure printing. Lithographic printing processes may use water as described above or they may discriminate on waterless bases. If waterless, discrimination is achieved by having a finite difference in surface energies between image and non-image areas. Special inks are necessary for this process.
Presently, most offset printing plates are prepared by analog prepress techniques. Photo type-setters and conventional cameras containing silver halide paper and film generate positive texts and photographs which are eventually converted to film negatives for plate preparation. This process is labor intensive and costly. The computer has now made it possible to convert this prepress operation from an analog process to a digital process, thus eliminating substantial labor and raw material costs. Total digitization of information also makes it possible to eliminate the film negative step in plate preparation and go directly to the plate. There are several means of digitally imaging a plate directly. Two of the most promising methods use lasers or ink jet technology. Using lasers to image plates directly is very costly because of the sophisticated optics and their controls. Time is also a constraint with lasers because of the necessity for raster scanning. A further drawback to laser imaging is the need for expensive, high-speed plate coatings, most of which are commercially unproven. Ink jet printing technology is much simpler and more cost effective. No optical beam deflectors or focusing optics are necessary. Ink jet heads can be easily integrated so that imaging time is minimized. The coatings on the offset plates needed in an ink jet plate maker can be standard negative working photopolymers with proven track records. High sensitivity silver halide coatings are not necessary.
Ink jet printing technology has been used to prepare printing plates. Several patent processes are described in the literature. Each of these processes has fundamental problems that makes it impractical for commercial use.
U.S. Pat. No. 4,003,312 was one of the first patents to recognize the advantages of using ink jet printing technology in a process for preparing a waterless lithographic printing plate. This patent discloses the use of an ink jet printing apparatus to deposit a background coating of silicone on an ink accepting substrate, the silicone being curable. The deposited silicone background coating must be cured at high temperatures.
U.S. Pat. No. 4,718,340 discloses a method for preparing a reusable planographic plate for lithography printing wherein a hydrophilic substrate is provided with a thin hydrophobic layer which is selectively removed. This process involves a multi-step plate preparation using hydrophobic organic acids and derivatives thereof. The treated surfaces are then selectively imaged with a spark discharge, or laser ablation technique. This process has limited run length capability similar to other spark discharge and laser ablation techniques.
U.S. Pat. No. 4,833,486 discloses a similar process, albeit not for a reusable plate. Here an ink jet printer is used to selectively image a hydrophilic surface with a hydrophobic true ink. The ink is hot and waxy so that it solidifies almost immediately on impact. The patent recognizes the problem of imperfections along the image edges and addresses this by applying the ink in a considerable thickness so that the marginal imperfections do not transfer ink. The process is thus a hybrid between lithography and letter press printing. Run lengths with such a system using "true" inks would be limited to several hundred at most. The image would wear rapidly through attrition and the imperfections in the jetted image would cause unacceptable images.
U.S. Pat. No. 5,312,654 discloses a method for making a printing plate wherein an ink absorbing layer is selectively imaged with a photopolymerizable composition using an ink jet printer. The ink absorbing layer prevents the ink from spreading and is removed after the ink is cured by exposure to actinic light, thereby exposing a hydrophilic surface where photopolymer has not been deposited. This process is impractical because the water soluble or alkali soluble coatings used as the ink absorbing layer have serious disadvantages since the ink jet imaged photopolymer sits on top of this layer. On a typical offset press, the use of an aqueous fountain solution would be disastrous for this plate. Additionally, the ink absorbing properties of this film limit control of dot or image formation and the resolution of fine details would still be problematic.
It is also possible to make a lithographic printing plate by using a mask in the form of an opaque material which is selectively applied according to one of several processes suitable for using an ink jet printer. According to one such method, a positive working surface, i.e. such as on a hydrophilic substrate of Al.sub.2 O.sub.3, a positive working light sensitive material, is selectively imaged with an opaque material using an ink jet printer. The non-imaged areas are then exposed to actinic light to render them soluble, whereupon the non-imaged areas are developed to expose the hydrophilic surface. Such a method is disclosed in Canadian Patent No. 2,054,320 of Nippon Paint Co. A disadvantage is that positive working compounds must be applied to considerable thickness and require longer exposures as well as longer development time. Using an opaque aqueous ink-jet ink would also produce dot and image growth problems.